Recent advances in the design and manufacturing of light emitting diodes (LEDs) have led to predictions that they will soon replace traditional lighting technology in a variety of illumination and display applications, offering the possibility of simultaneous modulation for communications. Because of cost and complexity concerns, the leading technology for white-light illumination involves the combination of a single LED with a phosphor coating. However, red/green/blue (RGB) LEDs can also be used for white-light illumination, as well as in applications that require colored, light. An RGB LED actually comprises three separate LEDs, each producing a different color. These three colors are perceived as a single color that can be varied by altering the relative average intensities of the. three LEDs (red, green, and. blue).
The main benefits that LEDs offer over traditional lighting technology include improved efficiency, durability, and life. However, it is has long been recognized that, since LEDs can be modulated, they present an additional opportunity for their simultaneous operation as communication devices. Indeed, impressive data rates have been reported using white light LEDs as reported in H. L. Minh, et al., “100-Mb/s NRZ visible light communications using a postequalized white LED,” IEEE Photonics Technology Letters, vol. 21,no, 15, pp. 1063-1065, August 2009 (hereby incorporated by reference). Because the communication system should not interfere with the primary application of the LEDs, additional constraints must be considered when designing the system. For example, modulated visible-light LEDs being used as illuminators should not appear to flicker, as this would undercut the quality of illumination and, worse, could pose a health hazard.
A variety of modulation and coding techniques have been considered for wireless optical communication, as described further in Z. Na, et ah, “The transmission performance of the MPPM modulation in indoor optical wireless communication based on white LED.” in Proceeding of SPIE, 2008 (hereby incorporated by reference) and J. Grubor, et al., “Broadband information broadcasting using LED-based Interior lighting,” Journal of Lightwave Technology, vol. 26, no. 24, pp. 3883-3892, December 2008 (hereby incorporated by reference). A use of color-shift keying (CSK), a modulation method recently proposed by the IEEE 802.15,7 Visible Light Communication Task Group, has been, described in IEEE 802.15.7 Visible Light Communication Task Group, (2010, Aug. 30). IEEE 802.15 Documents [Online] (hereby incorporated by referenced) (Available: https://mentor.ieee.org/802.15/documents?is group=0007).
By separately modulating the three emitters of an RGB LED, a wavelength diverse communication system can be obtained (exploiting a “red,” a “green,” and a “blue” channel). Amplitude-shift keying (ASK) and pulse-width modulation (PWM) can be modified to satisfy constraints on the perceived color of the LEDs. However, such constraints on perceived color and intensity, as well as system nonidealities including overlapping emission spectra, nonideal receiver filters, filter mismatch, and noise correlation in the three channels, complicates the design of optimal signaling constellations.